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Simple packet dump utility

This just reads packets from a tun device and prints them out on the console. It is known to work with Linux kernel 4.15. The intent of this program is to explore how packets can be read and dissected from the tap interface.

TUN/TAP frame format (for tun devices)

The TUN/TAP interface basically spits out raw frames to you. You have the option of having the kernel pre-pend a "packet information" header to this. It is useful when dealing with traffic that encapsulates IP datagrams, but can muddy the waters when other encapsulation protocols (e.g. 802.1Q VLANs) are thrown into the mix.

It turns out it's not so difficult to understand, but the standard TUN/TAP docs don't make it that clear how to decode that information.

Firstly, receiving frames is just done with the usual read() command. You use the usual mechanisms to wait for traffic to arrive on the file descriptor (i.e. select()), then when it arrives, you read() the entire packet in one gulp.

That implies your buffer will need to be big enough to hold the packet and the packet information header that TUN/TAP prepends.

Next, you'll want to look at the four byte information header. This is a struct tun_pi (see linux/if_tun.h).

  ----------- flags ------------   ------------ proto -----------
 /                              \ /                              \
+--------------------------------+--------------------------------+
|   Reserved for future use    |S|          Ethertype ID          |
+--------------------------------+--------------------------------+
                                |
                    TUN_PKT_STRIP

flags

This is a 16-bit register, given in host-endian format (i.e. little-endian on x86, etc) which is used to flag various conditions to the process reading the frame.

As it happens, there is just one flag that's applicable only when reading frames: TUN_PKT_STRIP. This flag is set when the packet you just read is too big for the buffer you've supplied.

The flags at this time is not used when writing frames back to the tun device: when writing frames, this should be set to 0x0000.

proto

This is the Ethertype ID of the data that follows. It is in network-endian format (i.e. big-endian), and a list of possible values can be seen in linux/if_ether.h, or alternatively on Wikipedia.

Notable ones that likely matter here:

  • 0x0800 (ETH_P_IP): Internet Protocol version 4
  • 0x86dd (ETH_P_IPV6): Internet Protocol version 6

Decoding the actual packet data

This will depend on the protocol that your frame encapsulates, the raw data for your frame will follow the 4 byte header you just read. Inspect the proto part of the header, then use that to choose an appropriate packet dissection algorithm.

This might be done by copying data to a struct iphdr (linux/ip.h) if it's an IPv4 payload or struct ipv6hdr (linux/ipv6.h) if it's IPv6. Alternatively you might choose to implement the dissector using your own data structures rather than using the Linux kernel ones.

tap differences

For tap devices, the format is the same. You get the same packet information header, and it'll even duplicate the "ethertype" field. However instead of an IP datagram, you'll get a raw Ethernet frame. Thus, you'll need to manage sending ARP requests (IPv4) or neighbour solicitation messages (IPv6), and manage a MAC address table.

If you cast the address of the data member of struct tundev_frame_t to a struct ethhdr*, you can extract the Ethernet frame protocol from that struct's h_proto member (again, big-endian) and the source/destination MAC addresses via h_source and h_dest respectively.

h_proto is once again, an Ethertype ID. Since this is layer 2, you will see other protocols here in addition to IPv4 and IPv6 mentioned above, e.g.

  • 0x0806 (ETH_P_ARP): Address Resolution Protocol
  • 0x8100 (ETH_P_8021Q): IEEE 802.1Q VLAN

Example output

With packet information (-no-pi omitted)

Raw IPv4 datagram

Flags: 0x0000  Proto: 0x0800
EType: 0x0800
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
IP Version 4  DSCP: 0  ECN: 0
Payload length: 84  Ident: 0x0000
Flags: 0x02  Frag Off: 0
TTL:    64     Proto:   1
CSum: 0xe178
Source IP: 172.24.0.1
Dest IP:   172.24.0.255
  98:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 45 00 00 54 00 00 40 00 40 01 e1 78 ac 18 00 01
  16: ac 18 00 ff 08 00 d9 84 0f fa 00 07 37 43 85 66
  32: 00 00 00 00 85 fd 0d 00 00 00 00 00 10 11 12 13
  48: 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23
  64: 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33
  80: 34 35 36 37 01 30 01 30 01 30 01 30 01 30 01 30
  96: 01 30

Raw ARP

Flags: 0x0000  Proto: 0x0806
EType: 0x0806
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
ARP HType: 1, PType: 2048
HLen: 6, PLen: 4
Operation: 0x0001
Sender HW Addr: ce:32:0b:4c:93:48
Sender Proto Addr: 172.24.0.1
Target HW Addr: 00:00:00:00:00:00
Target Proto Addr: 172.24.0.2
  42:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 00 01 08 00 06 04 00 01 ce 32 0b 4c 93 48 ac 18
  16: 00 01 00 00 00 00 00 00 ac 18 00 02 00 00 00 00
  32: 00 00 00 00 00 00 00 02 85 00

Raw IPv6 datagram

Flags: 0x0000  Proto: 0x86dd
EType: 0x86dd
To:    33:33:00:00:00:01
From:  ce:32:0b:4c:93:48
IP Version 6  Priority: 0
Flow Label:  0x0c19bf
Payload length: 64
Next header: 0x3a
Hop limit:   1
Source IP: fe80::cc32:bff:fe4c:9348
Dest IP:   ff02::1
 118:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 60 0c 19 bf 00 40 3a 01 fe 80 00 00 00 00 00 00
  16: cc 32 0b ff fe 4c 93 48 ff 02 00 00 00 00 00 00
  32: 00 00 00 00 00 00 00 01 80 00 03 13 d3 be 00 02
  48: 96 23 85 66 00 00 00 00 63 08 04 00 00 00 00 00
  64: 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
  80: 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
  96: 30 31 32 33 34 35 36 37 03 69 70 36 04 61 72 70
 112: 61 00 00 0c 80 01

IPv4 within 802.1Q

Flags: 0x0000  Proto: 0x0800
EType: 0x8100
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
802.1Q VLAN 123: Priority: 2 DEI: N Datagram protocol: 0x0800
IP Version 4  DSCP: 0  ECN: 0
Payload length: 84  Ident: 0x0000
Flags: 0x02  Frag Off: 0
TTL:    64     Proto:   1
CSum: 0xe176
Source IP: 172.25.0.1
Dest IP:   172.25.0.255
 102:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 40 7b 08 00 45 00 00 54 00 00 40 00 40 01 e1 76
  16: ac 19 00 01 ac 19 00 ff 08 00 0a 1f 0f 88 00 07
  32: ee 42 85 66 00 00 00 00 a7 d5 04 00 00 00 00 00
  48: 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
  64: 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
  80: 30 31 32 33 34 35 36 37 01 30 01 30 01 30 01 30
  96: 01 30 01 30 01 30

ARP within 802.1Q

Flags: 0x0000  Proto: 0x0806
EType: 0x8100
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
802.1Q VLAN 123: Priority: 2 DEI: N Datagram protocol: 0x0806
ARP HType: 1, PType: 2048
HLen: 6, PLen: 4
Operation: 0x0001
Sender HW Addr: ce:32:0b:4c:93:48
Sender Proto Addr: 172.25.0.1
Target HW Addr: 00:00:00:00:00:00
Target Proto Addr: 172.25.0.2
  46:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 40 7b 08 06 00 01 08 00 06 04 00 01 ce 32 0b 4c
  16: 93 48 ac 19 00 01 00 00 00 00 00 00 ac 19 00 02
  32: 00 00 00 00 00 00 00 00 00 00 00 02 85 00

IPv6 within 802.1Q

Flags: 0x0000  Proto: 0x86dd
EType: 0x8100
To:    33:33:00:00:00:01
From:  ce:32:0b:4c:93:48
802.1Q VLAN 123: Priority: 2 DEI: N Datagram protocol: 0x86dd
IP Version 6  Priority: 0
Flow Label:  0x0c19bf
Payload length: 64
Next header: 0x3a
Hop limit:   1
Source IP: fe80::cc32:bff:fe4c:9348
Dest IP:   ff02::1
 122:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 40 7b 86 dd 60 0c 19 bf 00 40 3a 01 fe 80 00 00
  16: 00 00 00 00 cc 32 0b ff fe 4c 93 48 ff 02 00 00
  32: 00 00 00 00 00 00 00 00 00 00 00 01 80 00 12 a1
  48: 0d 16 00 09 e9 3d 85 66 00 00 00 00 c8 01 03 00
  64: 00 00 00 00 10 11 12 13 14 15 16 17 18 19 1a 1b
  80: 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28 29 2a 2b
  96: 2c 2d 2e 2f 30 31 32 33 34 35 36 37 6b 69 73 68
 112: 69 c0 23 00 1c 80 01 00 00 00

Without packet information (-no-pi provided)

Raw ARP datagram

EType: 0x0806
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
ARP HType: 1, PType: 2048
HLen: 6, PLen: 4
Operation: 0x0001
Sender HW Addr: ce:32:0b:4c:93:48
Sender Proto Addr: 172.24.0.1
Target HW Addr: 00:00:00:00:00:00
Target Proto Addr: 172.24.0.2
  42:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 00 01 08 00 06 04 00 01 ce 32 0b 4c 93 48 ac 18
  16: 00 01 00 00 00 00 00 00 ac 18 00 02 00 00 00 00
  32: 00 00 00 00 00 00 00 02 85 00

Raw IPv4 datagram

EType: 0x0800
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
IP Version 4  DSCP: 0  ECN: 0
Payload length: 84  Ident: 0x0000
Flags: 0x02  Frag Off: 0
TTL:    64     Proto:   1
CSum: 0xe178
Source IP: 172.24.0.1
Dest IP:   172.24.0.255
  98:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 45 00 00 54 00 00 40 00 40 01 e1 78 ac 18 00 01
  16: ac 18 00 ff 08 00 47 55 13 80 00 09 ff 46 85 66
  32: 00 00 00 00 55 a1 04 00 00 00 00 00 10 11 12 13
  48: 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23
  64: 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33
  80: 34 35 36 37 34 35 36 37 01 30 01 30 01 30 01 30
  96: 01 30

Raw IPv6 datagram

EType: 0x8100
To:    33:33:00:00:00:01
From:  ce:32:0b:4c:93:48
802.1Q VLAN 123 Priority 2 DEI N
IP Version 6  Priority: 0
Flow Label:  0x0c19bf
Payload length: 64
Next header: 0x3a
Hop limit:   1
Source IP: fe80::cc32:bff:fe4c:9348
Dest IP:   ff02::1
 122:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 40 7b 86 dd 60 0c 19 bf 00 40 3a 01 fe 80 00 00
  16: 00 00 00 00 cc 32 0b ff fe 4c 93 48 ff 02 00 00
  32: 00 00 00 00 00 00 00 00 00 00 00 01 80 00 06 52
  48: 0b d4 00 03 00 3c 85 66 00 00 00 00 b3 9a 0e 00
  64: 00 00 00 00 10 11 12 13 14 15 16 17 18 19 1a 1b
  80: 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28 29 2a 2b
  96: 2c 2d 2e 2f 30 31 32 33 34 35 36 37 6b 69 73 68
 112: 69 c0 23 00 1c 80 01 00 00 00

802.1q carrying ARP

EType: 0x8100
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
802.1Q VLAN 123: Priority: 2 DEI: N Datagram protocol: 0x0806
ARP HType: 1, PType: 2048
HLen: 6, PLen: 4
Operation: 0x0001
Sender HW Addr: ce:32:0b:4c:93:48
Sender Proto Addr: 172.25.0.1
Target HW Addr: 00:00:00:00:00:00
Target Proto Addr: 172.25.0.2
  46:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 40 7b 08 06 00 01 08 00 06 04 00 01 ce 32 0b 4c
  16: 93 48 ac 19 00 01 00 00 00 00 00 00 ac 19 00 02
  32: 00 00 00 00 00 00 00 00 00 00 00 02 85 00

802.1q carrying IPv4

EType: 0x8100
To:    ff:ff:ff:ff:ff:ff
From:  ce:32:0b:4c:93:48
802.1Q VLAN 123: Priority: 2 DEI: N Datagram protocol: 0x0800
IP Version 4  DSCP: 0  ECN: 0
Payload length: 84  Ident: 0x0000
Flags: 0x02  Frag Off: 0
TTL:    64     Proto:   1
CSum: 0xe176
Source IP: 172.25.0.1
Dest IP:   172.25.0.255
 102:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 40 7b 08 00 45 00 00 54 00 00 40 00 40 01 e1 76
  16: ac 19 00 01 ac 19 00 ff 08 00 27 0d 13 57 00 05
  32: 92 46 85 66 00 00 00 00 df 16 08 00 00 00 00 00
  48: 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
  64: 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
  80: 30 31 32 33 34 35 36 37 01 30 01 30 01 30 01 30
  96: 01 30 01 30 01 30

802.1q carrying IPv6 traffic

VLAN is 123 (0x7b), priority 2.

EType: 0x8100
To:    33:33:00:00:00:01
From:  ce:32:0b:4c:93:48
 122:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
   0: 40 7b 86 dd 60 0c 19 bf 00 40 3a 01 fe 80 00 00
  16: 00 00 00 00 cc 32 0b ff fe 4c 93 48 ff 02 00 00
  32: 00 00 00 00 00 00 00 00 00 00 00 01 80 00 a3 f1
  48: c1 a1 00 02 8b 03 85 66 00 00 00 00 e2 66 01 00
  64: 00 00 00 00 10 11 12 13 14 15 16 17 18 19 1a 1b
  80: 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28 29 2a 2b
  96: 2c 2d 2e 2f 30 31 32 33 34 35 36 37 6b 69 73 68
 112: 69 c0 23 00 1c 80 01 00 00 00

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Example packet dump utility using tuntap

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